1. Field of the Invention
The present invention relates to an improvement of a pressure swing adsorption process which is a known process for separating a desired gas from a gas mixture such as air.
2. Description of Prior Art
A separation technique of gas mixtures in which separation of air is representative has been developed. As representative separation processes, there have been heretofore known a distillation process (e.g., low temperature separation process), an adsorption separation process and a membrane separation process. Among these processes, the adsorption separation process (especially, pressure swing adsorption process, generally abbreviated to PSA process) has the advantage that a desired product gas can be separated from a stock gas (feed gas) within a short time from the begining of the operation of the process and can be obtained with a high purity at a relatively high yield, as compared with the low temperature separation process and other processes, and has been employed for recovery of various gases from gas mixtures. The pressure swing adsorption process is shown, for example, in U.S. Pat. Nos. 3,430,418 and 3,564,816.
Recently, a compact system for producing nitrogen gas has been in great demand, for instance, for processing of electronic devices (e.g., LSI), production of various new ceramic materials and storage of substances related to biotechnology. A system for producing oxygen gas which is practically compact, lightweight and easy to handle has been also required for industrial and domestic uses.
It is known that the PSA process is advantageous in that desired gases can be easily obtained using a relatively compact and lightweight apparatus.
A typical PSA process generally comprises the following steps:
Step (1): introducing a gas mixture of two or more gases (such as air) under pressure into a cylinder having therein an adsorption column of an adsorbent which selectively adsorbs thereon gas component(s) other than a desired gas component from an inlet of the cylinder, to allow the adsorbent near the inlet to selectively adsorb the gas component other than the desired gas component and to form a zone of adsorbed gas in the column;
Step (2): keeping the introduction of the gas mixture into the cylinder under such conditions that the gas mixture flows through the column to move the front of the adsorbed gas zone forwards;
Step (3): collecting the desired gas component having passed through the column from an outlet of the cylinder;
Step (4): terminating the introduction of the gas mixture into the cylinder;
Step (5): terminating the collection of the desired gas component;
Step (6): discharging a portion of the pressurized gas mixture remaining in the cylinder; and
Step (7): returning a portion of the collected gas component back into the cylinder to flow through the column in the direction opposite to the direction of the movement of the gas mixture in the above step (2), to desorb the gas component having been adsorbed on the adsorbent of the adsorption column and purge away the desorbed gas component from the cylinder.
Steps (1) through (5) are performed for separation of the desired gas, and Steps (6) and (7) are preformed for regeneration of the column.
A representative procedure of the known PSA process is described in more detail by referring to FIG. 1 in the attached drawings.
FIG. 1 illustrates a known two cylinder system used for performing a pressure swing adsorption process. Each of two cylinders 11, 11a contains therein an adsorption column of an adsorbent. The cylinders 11, 11a have, respectively, inlets 12, 12a for a gas mixture to be separated and outlets 13, 13a for collecting a desired gas having been separated from the gas mixture in the respective cylinder. The inlets 12, 12a are connected to a source of gas mixture 14 by pipe lines via valves 15, 15a, respectively. The inlets 12, 12a are additionally connected to each other through a pipe line via valves 16, 16a. Between the valves 16, 16a, the pipe line has a waste gas line 17. The outlets 13, 13a are connected to a product gas reservior 18 by pipe lines via valves 19, 19a, respectively. The outlets 13, 13a are additionally connected to each other through a pipe line via valves 20, 20a. Between the valves 20, 20a, the pipe line has a branch pipe line which is connected to the product gas reservoir 18 via a pressure controlling valve 21.
The pressure swing adsorption process is performed in the system of FIG. 1 in the following manner.
A gas mixture (e.g., air and industrially produced gas mixture) is continuously supplied under pressure from the source of gas mixture 14 into the cylinder 11 from the inlet 12. The supply of the gas mixture is performed by opening the valve 15. During this procedure, the valves 15a, 16, 19, 20 are all closed. The gas mixture continuously introduced into the cylinder 11 advances forwards during which one or more gas components other than a desired gas component is adsorbed by the adsorbent of the adsorption column to form an adsorbed gas zone. Along with the advancement of the gas mixture, the front of the adsorbed gas zone moves forwards in the cylinder 11. When the pressure of the gas mixture in the cylinder 11 reaches a predetermined maximum level (at that time the front of the adsorbed gas zone resides midway between the both ends of the adsorption column), the valve 19 is opened to discharge the desired gas from the outlet 13 to direct it to the product gas reservoir 18. Then, just before the front of the adsorbed gas zone reaches the end of the column, the valve 15 is closed to terminate the supply of the gas mixture into the cylinder, and simultaneously, the valve 19 is closed. By a series of these procedures, a gas separation process is complete.
Subsequently, the adsorption column regeneration process is started. In this process, the valve 12 is first opened to discharge the pressurized gas remaining in the cylinder 11 through the waste gas line 17. The waste gas line 17 may be connected to a vacuum system (not shown) to more efficiently discharge the pressurized gas in the cylinder 11. By this discharging process, most of the gaseous portion remaining in the cylinder 11 is removed. However, most of the gas component adsorbed by teh adsorbent of the adsorption column remains in the cylinder because desorption of the adsorbed gas component hardly takes place by the above procedures simply relying on the difference of pressure between the interior of the cylinder 11 and the waste gas line. The desorption of the adsorbed gas component is then performed using a portion of the product gas (the desired gas component) previously collected in the product gas reservoir 18. For performing the desorption of the adsorbed gas component, a portion of the product gas is continuously returned into the cylinder 11 by opening the valve 20, placing the pressure control valve 21 under an operative condition. Thus returned product gas advances in the cylinder 11, desorbing the adsorbed gas component, and is discharged from the waste gas line 17. The gas may be discharged more efficiently by means of a vacuum system which may be provided to the waste gas line 17. The supply of the product gas is continued until most of the adsorbed gas component is desorbed and discharged from the waste gas line 17. The above procedure for desorbing the adsorbed gas component using a portion of the product gas is named a purging procedure. By a series of these procedures, the column regeneration process is complete.
In the pressure swing adsorption process, the set of the gas separation process and the column regeneration process is repeated to produce a required amount of the desired gas. In the first step in the next cycle, that is, the step for introduction of a gas mixture into the cylinder, a portion of the product gas may be returned to the cylinder for so controlling the pressure in the cylinder as to avoid abrupt change of the pressure.
When the gas separation process is performed in the cylinder 11, the column regeneration process is performed in the cylinder 11a. When the column regeneration process is performed in the cylinder 11, the gas separation process is performed in the cylinder 11a. Thus, the product gas (i.e., desired gas component) can be continuously collected in the reservoir 18.
In the pressure swing adsorption process, the pressure in the cylinder varies within a wide range to the extent from several Torr to several ten kg/cm.sup.2 G. Each step of the pressure swing adsorption process is carried out from several seconds to several minutes. A set of valves provided to the system are opened and closed automatically according to a predetermined program. One cycle of the pressure swing adsorption process, namely, one set of the gas separation process and the column regeneration process is performed from several ten seconds and several ten minutes. Therefore, the pressure change in the cylinder is apt to occur abruptly within a very short time. The abrupt change (or variation) of the pressure in the cylinder is apt to cause certain troubles. For instance, a neatly arranged adsorbent of the adsorption column may be disarranged, and the gas flows unsteadily in the cylinder. For example, a portion of the gas flows along the inner wall of the cylinder with insufficient contact with the adsorbent. Otherwise, a portion of the gas flows through a space formed by the disarrangement of the adsorbent with insufficient contact with the adsorbent. Such insufficient contact of the flowing gas with the adsorbent causes poor adsorption or poor desorption. Thus, efficiency of the gas separation process is adversely affected.
In order to obviate the troubles arising from the abrupt variation of the pressure in the cylinder, some measures have been proposed. For instance, it has been proposed to provide a manually operable valve to each of the automatically operable valve to more finely control the period of the gas flow. However, this measure has drawbacks in that the provision of the additional valves makes the gas separation system (apparatus) more complicated and further, fine control of the manual valves is required in the course of performing the gas separation process. It has been also proposed to employ an orifice having a fixed opening size in place of the manual valve. However, the provision of the orifice also makes the gas separation system more complicated and, further, no sufficiently effective improvement observed. Furthermore, it has been proposed to provide flow control valves to the separation apparatus. The provision of a number of flow control valves to the gas separation apparatus is very expensive and not advantageous in industry. Further, the maintenance of flow control valves to keep it under well controlled condition is not easy. Further, there is known the provision of ceramic balls in the cylinder at a space between its inlet and the adsorption column to protect the column from the abrupt pressure change of the introduced gas mixture. The provision of such barrier is not advantageous because resistance to flow of the gas mixture increases.